[WIP] Refactored user_name suggestion methods into a module

This could be a beginning to achieve a better design for User model as spoken in http://meta.discourse.org/t/little-refactor-over-user-model-to-avoid-global-complexity/7219/2

I think that it will be a good idea to divide the model into more simplistic little models and use them when necessary. 

I have tagged as WIP the PR but only to indicate that this is part of a bigger task, this refactor is really fully functional (the file user.rb has been also correctly indented).